The present invention relates to turbomachinery in general, and to turbochargers in particular. More specifically, the invention relates to a shaft assembly comprising a shaft joined to a titanium-aluminide turbine wheel, and to methods for making such a shaft assembly.
Aluminides are intermetallic compounds. Aluminide-based alloys are attractive for various applications because they combine superior high-temperature performance with low specific gravity. In particular, titanium aluminide alloys are of current interest for manufacturing turbine wheels for turbomachinery such as turbochargers. The gamma-phase titanium-aluminide alloys appear to be especially useful.
There are two basic forms of titanium-aluminide: Ti3Al alpha-2 phase containing between 22% and 39% aluminum, and TiAl gamma phase containing between 49% and 66% aluminum. There is also an orthorhombic form containing between 12.5% and 30% niobium, generally denoted as Ti2AlNb (O)-phase. In terms of material properties, Ti3Al alpha-2 phase has good high-temperature strength but poor oxidation and creep resistance. The alpha-2 phase is also extremely brittle and as such is difficult to weld effectively. The gamma phase has about one-third lower high-temperature strength than the alpha-2 phase but greater oxidation and creep resistance. The gamma phase is also less brittle and thus easier to weld. Even with the gamma phase, however, direct fusion-welding (e.g., by electron beam welding) of steel to titanium-aluminide is difficult to achieve without forming cracks in the weld zone.
Research in published literature has shown that solid-state cracking tends to occur in the weld zone as a result of high thermal stress and intrinsic brittleness of the welded microstructure. Some success in producing crack-free welds has been achieved by carefully controlling the welding parameters, notably the welding speed and cooling gradient, so as to reduce stress and the formation of brittle phases within the structure. Pre-heat and post-heat vacuum treatments have also been helpful in reducing or preventing cracking. It has been observed that solid-state cracking can be prevented by keeping the cooling rate below 300° C. per second and/or by reducing the intensity of the electron beam and keeping the weld speed below 10 mm per second. In practice, however, it can be difficult to control these welding parameters as accurately as would be necessary to ensure crack-free welds. Furthermore, even if crack-free welds are produced, there is still the possibility of post-weld cracking to occur when the welded component is introduced to an ambient atmosphere. It has been suggested that oxygen and hydrogen can be responsible for post-weld cracking.
In the production of turbochargers, the turbine wheel is generally welded to a steel shaft. Welding of titanium-aluminide to a different material such as steel further increases the difficulty of producing satisfactory welds because of the potential for uncontrolled creation of various intermediate materials in the weld zone. Thus, there is a need for a method for joining a titanium-aluminide turbine wheel to a shaft that avoids the above-noted difficulties.
Friction welding utilizes a rotary force to join a shaft blank to an as-cast wheel through a pipe-to-pipe joint. The resulting weld zone is brittle and requires a localized post-weld temper treatment to prevent fracture during machining of the finished component. The principal advantage of the electron beam welding process over friction welding is the ability to produce a parallel fusion zone with minimum heat input, which in turn reduces distortion and allows the welding of machine-finished components.
The development of electron beam welding and other high-energy density processes with rapid solidification characteristics has increased the commercial potential of welding dissimilar materials and resulted in novel and modified structures. Simultaneously, progress has been offset by problems within the fusion zone, boundaries, and heat-affected zones of both parent materials because of to phase transformations and cracking phenomenon.